In the adult human most hemoglobin is hemoglobin A (Hb-A) consisting of two alpha and two beta polypeptide chains. Certain individuals have an abnormal hemoglobin known as hemoglobin S (Hb-S) which results from the hereditary substitution of valine for glutamic acid in the sixth amino acid position in the beta polypeptide chains of hemoglobin. The proportion of Hb-S to Hb-A in such an individual depends upon whether the individual is a homozygous or heterozygous individual. The tendency toward sickling, that is, the formation of abnormally shaped erythrocytes in which the erythrocytes assume a sickle shape, depends upon the amount of Hb-S in the erythrocyte and the level of oxygen tension. Erythrocytes with 100 percent Hb-S sickle at physiological oxygen tensions, however as the amount of Hb-A increases and Hb-S decreases progressively lower oxygen tensions are required to induce sickling. The homozygous individual has 80 to 100 percent of the hemoglobin in the Hb-S form and sickling occurs at ordinary oxygen tensions. Such individuals are said to have sickle cell disease. Heterozygous individuals are said to possess sickle cell trait since only 25 to 40 percent of their hemoglobin is Hb-S, and sickling occurs only at unusually low oxygen tensions.
The presence of sickled erythrocytes can have severe implications since sickled erythrocytes encounter mechanical difficulties in moving through small vessels and the consequent stasis and jamming of these cells can lead to thrombosis and tissue anoxia. In addition, because of the sickled erythrocytes' increased mechanical fragility, hemolysis results, S. L. Robbins and M. Angell, "Basic Pathology", W. B. Saunders Company, Philadelphia, London, Toronto, 1971, pp. 127 and 282.
A treatment or test in which the sickling of red blood cells prone to sickle (sickle erythrocytes) is inhibited or reversed would be useful in the treatment of afflicted individuals or for the study of the sickling phenomenon.